A variety of state measuring apparatuses have been used in a wide field of applications for taking measurements by converting physical quantities into electric quantities. In automotive vehicles, in particular, states of various portions are accurately detected by using many sensors, so that control operations are carried out to optimize the operating performance, fuel efficiency and harmony with the environment.
The measuring apparatuses must satisfy the requirements of wide detection ranges and high resolution in addition to the accuracy of measurement. To satisfy such requirements, in the case of a measuring apparatus, the gain of an amplifier is adjusted depending upon the content that is measured, and an offset is adjusted depending upon the gain adjustment.
One conventional measuring apparatus that measures pressure is disclosed in JP 2003-273673A and schematically illustrated in FIG. 1A. In this figure, a sensor chip 1 is a bridge circuit constructed with four resistors including a resistor, which varies its resistance depending upon the pressure being applied and measured. In the bridge circuit, tow resistor rows each having two resistors connected in series are arranged in parallel, and a voltage is applied to the resistor rows. A difference of the voltage is detected across the two connection nodes of the resistors connected in series. When the resistance of one resistor varies depending upon the pressure, the difference in the voltage varies across the two connection nodes of the resistors connected in series, and the variation thereof is output after being amplified by an amplifier 2. To convert the voltage into a digital signal, the output is converted into a digital data through an analog/digital (A/D) converter. It is also allowable to measure a difference in the pressure by so constructing the two resistors of the sensor chip 1 that the resistances thereof vary depending on the pressure on the two portions that measure the difference in the pressure.
The amplifier 2 is capable of varying is gain using a gain control 3, and the gain is adjusted depending upon a pressure value being measured. The amplifier 2 is further capable of varying the zero level of its output by using an offset control 4, and adjusts the zero level at the time of initialization, when the gain is varied and when the temperature is varied. The gain control and the offset control are calibrated at all times by holding the sensor chip 1 under a predetermined condition or by inputting a signal for calibration.
The measuring apparatus may be capable of effecting the calibration at all times, but a sensor incorporated in a device is not capable of effecting the calibration. Further, to effect the gain adjustment or the zero level adjustment, a circuit having a variable resistor must be added. This increases the cost. Therefore, a unit incorporated in the device is constructed with no gain control nor offset control.
A pressure state measuring apparatus for measuring pressure may be constructed as shown in FIG. 1B. The state measuring apparatus has a pressure sensor 5 including the sensor chip 1 shown in FIG. 1A. An amplifier 6 is provided to amplify a voltage difference across the two connection nodes of the sensor chip 1 and outputs it as Vout. The gain and the offset of the amplifier 6 are fixed and not variable. A low-pass filter 7 is provided to remove high-frequency components, which are noise components in the output Vout of the pressure sensor 5, and an A/D converter 8 is provided to convert the output of the filter 7 into a digital data. The digital data are sent to a control unit which executes a variety of controls based on the values detected by the sensor.
Devices that use a temperature sensor or a humidity sensor basically assume the similar construction. The pressure sensor is an electric element which varies the resistance depending upon the physical quantity (pressure) to be measured. Depending upon the cases, however, there will be used an electrostatic capacity or an electronic element that varies the inductance.
The resolution of the construction illustrated in FIG. 1B is determined by a value obtained by dividing a dynamic (measuring) range of the amplifier 6 by the number of discrimination levels specified by the number of bits of the A/D converter 8. When the noise level of the measuring system is greater than the resolution, the resolution is specified by the noise level. To obtain a wide dynamic range while decreasing the resolution, it is considered to increase the number of bits of the A/D converter. However, the cost of the A/D converter sharply increases with an increase in the number of bits thereby increasing cost.
It has further been attempted to provide an oscillation circuit with an electric element which varies the electric characteristics depending upon the physical quantity to be measured, so that the oscillation frequency (period) of oscillation signals varies depending upon a change in the physical quantity to thereby measure the physical quantity by detecting the oscillation frequency. The oscillation frequency can be detected by, for example, counting the number of pulses of oscillation signals in a predetermined period. Further, the length of the oscillation period of oscillation signals can be detected by counting the pulses in a predetermined period. The effect of noise can be decreased by converting the physical quantity into a change in the oscillation frequency instead of converting it into an analog intensity signal. Here, the sensor, which varies the frequency (period) of oscillation signals in response to a change in the physical quantity, is called an oscillation-type sensor.
JP 9-147283A discloses a long-distance transmission system for executing signal transmission over a distance by converting a physical quantity into a frequency (period) of oscillation signals by using an oscillation-type sensor, which varies an electric resistance or an electrostatic capacitance depending on an amount of water content, temperature or pressure and a CR oscillation circuit, which varies the oscillation frequency depending upon an electric resistance or an electrostatic capacity of the sensor. Further, JP 9-43078A discloses a construction that converts a capacity of an electrostatic capacity-type sensor into an oscillation frequency. Further, IP 2000-55954A discloses a construction that converts a resistance into an oscillation frequency.
In the conventional state measuring apparatuses for detecting the physical quantity by utilizing the above oscillation-type sensors, the range of variation in the oscillation frequency is designed for each of the state measuring apparatuses to meet the range of variation in the physical quantity to be measured, resolution and characteristics of the electric element, and is fabricated in the form of a hardware. Therefore, the dynamic range and resolution are fixed and cannot be, usually, changed.
In a unit, such as an engine control unit for an internal combustion engine, that executes processing by reading outputs from many sensors, therefore, detector circuits are provided for detecting various oscillation frequencies (periods) to meet various oscillation-type sensors. Namely, a number of different detector circuits are required, thus increasing cost and size.
In addition, the state measuring apparatus requires a wide dynamic range and a high resolution. It is therefore difficult to satisfy both of these requirements even by using the detection circuits in the oscillation-type sensors. For example, it is possible to take a measurement over a wide dynamic range maintaining a high resolution if the period for counting the oscillation signals is lengthened. However, an increase in the number of bits of the counter necessarily causes an increase in the size of the counter, in the size of the detector circuit and an increase in the cost.